Radiant energy sensitive device



March 22, 1960 -F. H. NICOLL RADIANT ENERGY SENSITIVE DEVICE Filed D80. 30, 1954 .......nla

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nADIAN'r ENERGY SENSITIVE DEVICE Frederick H. Nicoll, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application December 30, 1954, Serial No. 478,815 15 Claims. (Cl. Z50-211) This invention relates to improvements in radiant energy sensitive devices.

While the devices to be described have particular utility in detecting or reproducing radiation or images falling in the visible spectrum, the principles of this invention apply equally as well to other types of radiation, for examples X-rays, infrared radiations, ultra-violet light radiation and the like. Therefore, for simplicity, the term radiant energy will be understood to encompass -rays as well as visible and invisible radiations. v

This invention is directed to improving the sensitivityv of'devices such as, for example, certain types of picturereproducers which for design reasons utilizea thick layer of photoconductive material as a light sensitive cell. In such devices the photoconductive layer may be so thick that light is unable to penetrate completely through the layer. Hence, under excitation, there exists through the thickness of the photoconductive cell a low impedance 'layer where the light penetrates and a series high impedance layer which the light does not reach and which remains unsensitized. -The presence of such an unsensitized layer greatly detracts from the overall performance of the device, especially its sensitivity and, if the device is a picture reproducer, its brightness and ampliiication.

One type of picture reproducing device to which this invention applies utilizes a projection screen constituted of a layer of photoconductive material and a layer of electrolumnescent material sandwiched between two transparent electrodes. The electrolumnescent material is constituted of particles of luminescent phosphor material embedded in a dielectric and having theV property of emitting light under the influence of an electric field. A voltage is applied across the two electrodes to provide an electrical circuit in which the impedances of the two layers are in series. It has been necessary to make the thickness of the photoconductive layer much greater than that of the electrolumnescent layer so that the impedance of the'photoconductive layerin the dark will be many times that of the electrolumnescent layer, and a greater fraction of the applied voltage will be developed across the photoconductive layer. Under these conditions, the voltage across the electrolumnescent layer is below the threshold voltage required to cause visible luminescence and no light is emitted from the electroluminescent layer. When light falls on the photoconductive material, its impedance is lowered in the areas where the light strikes, and a greater fraction of voltage is applied across the electrolumnescent material directly. in contact with the illuminated areas of the photoconductor. The electrolumnescent material is thus caused to emit light.

The light emission increases with an increase in the voltage applied across the electrolumnescent layer so that a halftone image can be projected on the photoconductive material and a corresponding halftone image will be reproduced on the electrolumnescent material. I n addition, it the input image is of low intensity, the

`continuous photoconductive Yplate or electrode 12,

2,929,934 Patented Mar. 22, 1 960 output image can be an amplified light image. Also an invisible image, such as an infrared image or an ultraviolet image can be converted or reproduced in visible form.

In some devices, theelectrolurninescent layer is about 1 mil in thickness, and to achieve the required voltage distribution between the two layers in the dark, the photoconductive layer is made about l0 mils thick. Such a thick photoconductive layer is notv very eiiicient because substantially all the light is absorbedl within a short depth of the photoconductor and the lower depths remain relatively unillutninated. The problem, therefore, is to devise some means whereby the effective depth ofthe photoconductor excited by the incident light is greatly increased, in order to provide produce greater light output from the electrolumnescent layer.

it is, therefore, an object of this invention to provide a radiant energy sensitive device of improved sensitivity.

it is a further object to provide an improved photoconductive cell having a novel structure which permits light to penetrate through a greater effective depth.

'It is a further object to provide an improved light image reproducer 'of the kind including as one of its elements a layer, said limage reproducer having greatly improved sensitivity and amplication.

It is another proved method tive device. Theforegoing and other objects maybe'achievedin accordance with the invention by providing in-a light sensitive sheet-like member, regions which are more .transparent to incident light than otherregions. For example, this may be accomplishedy by providing in a continuous layer of photoconductive material having an overall thickness which is greater than a predetermined light penetrating thickness for said material, regions which have a thickness less than said light penetrating thickness. More specilically the light sensitive cell is formed by spacing an apertured electrode from a support plate and filling the intervening space with a mixture of photoconductive powder, a plastic binder and a solvent for thebinder. When the mixture dries, the evaporation of the solvent allows the photoconductive powder and binder underlthe apertures to contract down towards the support plate. Depressions are left in the surface of the cell which exposela substantial depth of photoconductive material to the action of incident light.

In the single sheet of drawings:

Fig. l isa planview showing a light sensitive cell constructed in accordance with the invention. y

Fig. 2 is a section of Fig. 1 taken along lines 2 2. i

Fig. 3 is a sectional view of a light image reproducer constructed according to the invention.

Referring to Figs. l and 2 in detail there is shown a photosensitive cell constructed in accordance with the invention. The cell it) comprises a conductive support I an insulating spacer 14, an apertured light permeable electrode 16 spaced from the plate object of this invention to provide an imn of manufacturing a radiant energy sensi- 12 and a continuous layer of photoconductive, material 18 intermediate the two electrodes l2 and 16. The electrode 16 may be a thin metal plate provided with apertures, or it may be a wire mesh, or grill structure. The electrodes l2 and 16 are connected in series with a voltage source 20 and a load 22. A y v As seen more clearly in Fig. 2, the photoconductive layer 18 is formed with depressions or voids extending from the apertures in the electrode 16, and with their bottoms close to the support plate 12.y v There is thus produced in the.photoconductive-layer 18,' regions, at the depressions, which are more transparent to incidentlight tiveilayers formed vwithout the depressions.

- be ethyl cellulose.

light incident on the apertures electrode` side, the light will ,passinto the vlighttransparent regions :andilluminate the photoconductive layer through a greaterxeliective-depth.

Provision of lthese light transparent regions avoids .the limitations of reduced sensitivity of thick Vphotoconduc- Solid layers of fa thickness greater than va given light penetration .thickness have the serious limitation in that ythe lower regions of the layer are not reached by the light. Thek impedance lremains highin these regions .and the sensitiv- .ity 'of the .cell is yconsiderablyrreduced. ByV means ofA this invention, then, the overall thickness ofthe layer .18 may be many times the order of thickness in which substantially all light is absorbed, and yet-the light will reach kand excite the lower depths ofthe layer. ',The depressions must be deep enough, of course,'so fasinot .to leave a thickness of photoconductive material below them which is greater than can -be penetrated b'y the To Sfabricate the cell 10, when the'electrodeis in the form-cfa mesh, the mesh is strethed 'taut across the'top of the spacer 14, which rmay be 15 'mils thick. Ihe

photoconductive material `18, vtor example, crystalline ...Cadmium Asuliide powder, preferably tiner than '325 mesh, ,is .mixed with a l0 percent solution of ethyl cellulose binder in Aamyl alcohol with ra ratio of two parts of powder to kone .part of solution by weight. The` mixture is then squeegeed across the mesh with a rubber or metal strip sothat the powder-solution mixture fills the interstices of the mesh. When the solution dries, the evaporation of the solvent allows the binder-powder mix to contract `down into the :mesh in the desired manner as shown -in Fig. 2.

The invention is not "limited to vthe specic materials mentioned above. Any othermaterial whose impedance is variable in response to radiant energy excitation may yhe used in place of the cadmium sulfide. For example, cadmium selenide or lead suliide may be used. Other suitable plastic bin :lers are methyl-methacrylate and polystyrene. ySuitable vsolvents for these materials are ethyllene dichloride, and toluene, respectively. A tcasting or self setting organic resin may also be used, in which case it is also diluted with a volatile solvent. The vstrength of the binder-solvent solution and vthe powdersolution weight ratiomay `be varied .depending on the ,materials used and the vamount of depression desired.

Y'The above technique maybe vused to ,fabricate 4the vpicture reproducer shownin Fig..'3. ,A transparent support Aplate 24 of -plastic' or glass, having a Athin transparent conductive coating 26 of tin chloride, for example, is coated with a layer 28 `lof electro-luminescent phosphor particles Vin plastic. The .phosphor may' be zinc sulfide activated with copper kand Ythe `plastic: may

An insulating spacer 30fis then supported on the phosphor layer, a wire mesh 32is stretched taut over the spacerl) may be formed adjacent to-the phosphor layer 28in the manner described for kthe cell 10 of Figs. l and 2. In one example, the phosphor layer was l to 2 mils thick and the insulating spacer 30 was l5 mils thick. `If desired, ,a light opaque insulating layer 35 may Ahe interposed between the A,photoconductive layer 34 and the phosphor "layer 28. The layer 35 ,may bega thin ,layer o'fblackllacquer or `carbon black vparticles mixed with insulating materia.

To operate the device of'Fig.'3 as a light amplifier or picture reproducer, a voltagesource 3'6, preferably Valternating current, is connected across the. film 26 .and mesh 32 which serve as electrodes. `'Consider the relative thickness of the'photoconductive layer 34 v and .the phosphor layer 28 to be such that the series impedance of the photoconductivelayer in the dark or unexcitedcondition .tozzbegofthe order .of ten times that of ,the ,phosphor layer. Since these impedances are in series with `the supply voltage 36, the apportioned voltage appearing across the photoconductive layer will be ten times the voltage appearing across the phosphor layer. Also, consider the supply voltage to be adjusted so that the magnitude of the voltage appearing across the phosphor layer is justbelow the threshold value required to cause the phosphor to luminesce. Under these conditions then, with no incomingA radiation incident on the photoconjductorVno'light is emitted fromthe phosphor.

Consider then anVV amount of light falling on an elemental area `of the photoconductor, for example, through one of the apertures in the mesh electrode 32 so that -the photoconductor, which is a function of the intensity ofthe incident light, causes a corresponding increase in the voltage appearing across the phosphor in an ,directly :adjacent to Vthe excited photoconductor. phosphor is thus caused to emit light in this area due area The to an'increase yat lthis localized area in the voltage above the threshold value Lrequired for luminescence. Because the intensity of the Ylight emitted from the phosphor is proportional to the Vmagnitude of the field developed across the phosphor, A.it is readily seen that an image with half and a photoconductive layer 34 `tones can be reproduced on the phosphor surface which is a replica of theimage incidenten the photoconductor. 'The :opaque layer 35, if used, will prevent light feedbfack from the-phosphorl layer to the photoconductive layer. The layer 35 maybe omitted, and light feedback-will vlue-prevented if the light emitted from the phosphor layer youtside that portion of the spectrum to which the photo- .conductive ilayer is sensitive. it may be advantageous,

under certain circumstances, to omit the layer 35 and allow light feedback to enhance the amplification or to store a Vreproduced image even after the incident image is removed.

As indicated previously, if a uniformly thick solid Vlayer of photoconductor is used in a picture reproducer lof this kind, the incident light will not reach the lower `depths ofthe photoconductive layer. Hence the photoconductor will beinetiicient in transferring the maximum desired proportion vof `the total .supply voltage to the vphosphor ,layer for any .given level of incident light. Witha device constructed according to Fig. 3, however, in which thephotoconductive layer is provided with lig'nt transparent regions, this `handicap is overcome. These lighttransparent ,regions-allow vlight to excite the photoconductor through la greater effective depth.

What is claimed is:

l.- A radiant energy sensitive device comprising a sheetlike member ,including a continuous layer of a material having .a variableimpedance characteristic in response to radiant energy excitation, lsaid member including a plurality of regions which .are more transparent to incident radiant 4energy than other regions of said member.

.'2. A light sensitive device comprising a continuous layer of ,photo-conductive material, said layer including Ya plurality olf-regions Vwhich are A.more transparent to incidentlightthan other jregions.

3. `A "light-sensitive .device comprising a continuous layer .of .photo-conductive material having an overall thickness .which is greater than a predetermined light penetrating thickness for said material, said layer including a plurality of regions which have a thickness less than said flight ,penetrating thickness.

4. A radiant energy sensitive device comprising a continuous layer of amaterial having a variable impedance characteristic in responseV toradiant energy excitation, said layerincluding a plurality ofY regions which are more falls within a range of wavelengths lying transparent to incident radiant energy than other regions of said material, and a pair of electrodes in contact with opposite sides of said layer.

5. A light sensitive device comprising a continuous layer of photo-conductive material, said layer including a plurality of regions which are more transparent to incident light than other regions, a source of voltage, and means for connecting said voltage source to said layer.

6. A light sensitive device comprising a continuous layer of photo-conductive material having an overall thickness which is greater than a predetermined light penetrating thickness for said material, said layer including a plurality of regions which have a thickness less than said light penetrating thickness, and a pair of electrodes in contact with opposite sides of said layer.

7. A light sensitive device comprising a conductive base member, an apertured electrode spaced from said base member and a continuous layer of photoconductive material therebetween, said layer having a plurality of depressions directly under the apertures of said electrode and which extend substantially through the entire depth of said la er, whereby light incident on said apertured electrode will illuminate said depressions and thus excite said photoconductive material through a greater effective depth.

8. A light sensitive device comprising a conductive plate electrode, a mesh electrode spaced from said plate, and a continuous layer of cadmium sulde powder mixed with a plastic binder between said respective electrodes, said layer having depressions directly underneath the apertures of said mesh electrode and which extend substantially through the entire depth of said layer, whereby light incident on said mesh electrode will illuminate said depressions and thus excite said photoconductive material through a greater effective depth.

9. A radiant energy reproducing device comprising a sheet-like member including a continuous layer of material having a variable impedance characteristic in response to radiant energy excitation, said member including a plurality of regions which are more transparent to incident radiant energy than other regions of said member, a layer of electroluminescent phosphor material electrically in series with said member, and a pair of electrodes in contact with opposite sides of said member and layer` A radiant energy reproducing device comprising a transparent base, a transparent conductive coating on said base, a layer of electroluminescent phosphor on said conductive coating, a continuous layer of photoconductive material on said phosphor layer, said photoconductive layer including a plurality of regions which are more transparent to incident light than other regions, and a light permeable electrode on said photoconductive layer.

11. A radiant energy reproducing device comprising a transparent base, a transparent conductive coating on said base, a layer of electroluminescent phosphor on said conductive coating, a continuous layer of photoconductive material on said phosphor layer, said photoconductive layer including regions which are more transparent to incident light than other regions, a light permeable electrode on said photoconductive layer, and a light i opaque insulating layer disposed between said phosphor and said photoconductive layer.

12. A radiant energy reproducing device comprising a transparent base, a transparent conductive coating on said base, a layer of electroluminescent phosphor on said conductive coating, a continuous layer of photoconductive material on said phosphor layer, said photoconductive layer having an overall thickness which is greater than a predetermined light penetrating thickness for said material, said layer including a plurality of regions which have a thickness less than said light penetrating thickness, and light permeable electrode means on said photoconductive layer.

13. A radiant energy amplifier comprising a transparent support member, a transparent conductive lm thereon, a layer` of electroluminescent material on said lm an apertured electrode spaced from said electroluminescent layer, and a continuous layer of photoconductive material between said electrode and said electroluminescent layer, said photoconductive layer having a substantially greater dark impedance than said electroluminescent layer anda correspondingly greater overall thickness, said photoconductive layer having depressions directly under the apertures of said electrode and which extend substantially through the entire depth of said photoconductive layer, whereby light incident on said apertured electrode will illuminate said depressions and thus excite said photoconductive material through a.A

greater effective depth.

14. The invention according to claim 3, wherein said photoconductive material consists of crystalline cadmium sulfide powder mixed with a plastic binder.

15. A light sensitive device comprising a conductive base member, insulating spacing means on said conductive base adjacent to the edges thereof, an apertured electrode on said spacing means and spaced from said base member thereby, and a continuous layer of photoconductive material in the space between said base member and said electrode, said layer having depressions directly under the apertures of said electrode and which extend substantially through the entire depth of said` layer, whereby light incident on said apertured electrode will illuminate said depressions and thus excite said photoconductive material through a greater eiective depth.

of the Optical Soc. of America,'vol. 44, No. 4, April 1954, pages 297 to 299. 

